1. Technical Field
The present invention relates to micro electro mechanical system (MEMS) devices equipped with structures such as movable and fixed electrodes on a silicon substrate using semiconductor fabrication processes and to fabrication methods of such MEMS devices.
2. Related Art
Development in microfabrication technology raises interest in electromechanical devices, or so-called MEMS devices, such as resonators, filters, sensors, and motors. Each of these devices has minute structures composed of a movable electrode and a fixed electrode fabricated by semiconductor fabrication processes. Because semiconductor fabrication processes are used in the fabrication of the MEMS device, a complex device combining the MEMS device with, for example, a complementary metal oxide semiconductor (CMOS) can be produced. Therefore, the MEMS device is also required to meet expectations to respond to today's growing demands for miniaturization and higher performance of electronic apparatuses.
Due to the demands for miniaturization and higher performance of electronic apparatuses, MEMS devices are expected to perform with a higher precision current value control at an electrically higher speed. Therefore, it is requested that the structures and wiring making up the circuitry of the MEMS device have lower resistance. For example, a large insertion loss of a MEMS device, such as a radio frequency MEMS (RF-MEMS) that operates at a high frequency band, directly affects the characteristics of the MEMS device. Therefore, it is necessary to keep the resistance value of the entire circuitry of the MEMS device as low as possible.
One exemplary method for fabricating the MEMS device is as follows. First, a fixed electrode and a movable electrode with a portion thereof being on a sacrificial layer are provided on a semiconductor substrate made of, for example, silicon (Si). Then, a wiring layered part containing wiring is provided on the fixed electrode and the movable electrode. Then, a portion of the wiring layered part and a portion of the sacrificial layer are removed by etching (release-etching) so as to release the movable electrode, thereby providing the movable electrode in a mechanically movable state.
The wiring in the circuitry of the MEMS device is generally provided, as in common semiconductor fabrication, by depositing metal such as aluminum (Al) through sputtering, chemical vapor deposition (CVD), or vacuum vapor deposition, and then by patterning. The wiring therefore has a low resistance. In contrast, the structures of the MEMS device including the movable electrode and the fixed electrode need be treated in order to lower the resistance of silicon, which is a semiconductor, after the silicon deposition and patterning. A known treatment method to lower the resistance of the structures made of silicon is to implant impurity ions such as phosphorous ions (e.g., 31P+) into the silicon film to provide a diffusion layer (e.g., see JP-A-2004-221853).
Another method to further reduce the resistance of the structures is depicted in, for example, JP-A-2004-221853 or JP-A-2001-264677, in which a metal is deposited on a silicon film through sputtering, CVD, or vacuum vapor deposition, followed by annealing at a high temperature so as to silicidize, that is, diffuse and alloy the silicon abutting on titanium which is the metal. The portion silicidized with titanium (TiSi), for example, has a specific resistance of approximately 10−5 Ωcm, and this is about one hundredth of that of a diffusion layer produced using impurity ion implantation.
As described above, by the method of producing a diffusion layer using impurity ion implantation into the structure made of silicon, it is difficult to reduce the resistance value of the structures to a resistance value required for the MEMS device to perform at a high frequency band. In contrast, the method of silicidizing the silicon structure is effective in drastically lowering the resistance of the structures. However, depending on the type of metal for silicidization, the silicide part may dissolve in the etchant when etching and removing the portion of the wiring layered part and the portion of the sacrificial layer to release the movable electrode. If the silicide part is dissolved, the resistance value may increase, and the thinned structures may lose its mechanical strength. As a consequence, the electrical and mechanical characteristics of the MEMS device may fluctuate, and the desired characteristics may not be attained.